1. Field of the Invention
The present invention relates to an illumination optical system including a lens array that splits a light beam emitted from a light source into multiple light beams and also relates to an image projection apparatus including the illumination optical system.
2. Description of the Related Art
An illumination optical system heretofore known includes a first fly's eye lens, which splits a light beam emitted from a light source into multiple light beams, and a second fly's eye lens, which includes multiple lens cells corresponding to the first fly's eye lens, in order to uniformly and efficiently illuminate an illumination surface of, for example, a liquid crystal display element.
The English abstract of Japanese Patent Laid-Open No. 10-115870 (Patent Document 1) discloses an illumination optical system in which decentered lenses are adopted as lens cells that constitute a first fly's eye lens or a second fly's eye lens to improve parallelism of light beams and reduce the amount of light loss in the illumination optical system.
US 2003/0174294 (Patent Document 2) and the English abstract of Japanese Patent Laid-Open No. 2003-090981 (Patent Document 3) each disclose an illumination optical system in which thicknesses of lens cells, which constitute a first fly's eye lens or a second fly's eye lens, are made different from one another in a stepwise manner in accordance with their amounts of decentering so that curved surfaces of the decentered lens cells are substantially continuous with one another.
However, there is a problem with the technology disclosed in Patent Document 1 in that a shadow is generated in an illumination area because the curved surfaces of the decentered lens cells are discontinuous and steps exist between the decentered lens cells (FIG. 11A).
In the technologies disclosed in Patent Documents 2 and 3, thicknesses of the lens cells are made different from one another so that the curved surfaces of the decentered lens are continuous with one another. However, the technologies have a problem in that, by making thicknesses of the lens cells different, principal points of the lens cells are displaced in an optical axis direction and consequently split light beams do not converge at target convergent points (FIG. 11B).
When decentered lenses are adopted as the lens cells constituting the first fly's eye lens and curved surfaces of the lens cells are continuous with one another as illustrated in FIG. 12, positions at which split light beams split by the lens cells of the first fly's eye lens maximally converge are displaced from principal points of lens cells of the second fly's eye lens. The reference signs x in FIG. 12 schematically indicate how much principal points of lens cells are displaced from a principal point of the lens cell on the optical axis of the illumination optical system, and how much focus points of split light beams from the lens cells are displaced from a focus point of a split light beam from the lens cell on the optical axis. Due to the displacement, some of the split light beams that are supposed to be incident on the corresponding lens cells are unintentionally made incident on adjacent lens cells and illuminate an area outside of an effective area of a liquid crystal display element. Accordingly, some amount of light loss occurs.